1) Field of the Invention
The present invention relates to a coloring agent that metachromatizes sensitively and reversibly in response to temperature changes.
2) Description of the Related Art
A reversible thermochromic material is a material that metachromatizes near a prescribed temperature and utilizes this characteristic in such fields as articles requiring a suitable temperature or a warning or danger temperature indication, and merchandise to enjoy changes in colors, and can be used for example in relation to frozen, refrigerated and heated food and beverages, stationeries such as writing instruments and postcards, clothing, toys and the like. Heretofore, several materials or compositions were known as reversible thermochromic material such as (1) materials utilizing a crystal of a metal complex, (2) materials utilizing a cholesteric liquid crystal, (3) compositions having as three constituents thereof an electron donative color former, a chemical compound having a phenolic hydroxyl radical and a chemical compound having an alcoholic hydroxyl radical, (U.S. Pat. No. 2324671, U.S. Pat. No. 3560229, U.S. Pat. No. 4028118), (4) compositions having as three constituents thereof a chemical compound which belongs to the class of phthaleins or floresceines, an electron donative organic nitrogen compound, and a chemical cosmpound selected from alcohols or amides (Japanese Patent Laid-open No. 65772/1982), and (5) compositions containing a polyhydroxy compound, an alkali metal salt of boric acid and a pH indicator.
Of these reversible thermochromic materials, (1) the one using a crystal of metal complex is superior in heat resistant and light resistant properties and is partly utilized for industrial use, but its metachromatizing temperature is higher than room temperature and it further has the defect that its chromic hue is limited. (2) Cholesteric liquid crystals are used for thermometers, ornaments and daily necessaries, but its metachromatizing temperature cannot be freely selected and also it is expensive and has the defect that its metachromatism life is short. (3) The combination of an electron donative color former and 2,2-bis-(4-hydroxyphenyl) propane (hereafter called bis-phenol A) is widely used in stationeries, toys, clothing and the like, but its usage is limited because of its poor light resistance, (4) a composition having as three constituents thereof, viz., a chemical compound of phthaleins or fluoresceines, an electron donative organic nitrogen compound and a chemical compound selected from alcohols or amides, has a limited chromic hue, and (5) the combination of a polyhydroxy compound, an alkali metal salt of boric acid and a pH indicator has the problem that its usage is limited because its metachromatism temperature is high and some time is required for it to chromically respond to temperature change.